In diesel engine fuel systems, it has long been necessary to filter fuel to very fine levels prior to admission to the fuel injection pump. With the introduction in recent years of higher pressure systems (e.g. the High Pressure Common Rail or HPCR systems) much finer fuel pump and injector clearances have been developed in order to achieve the very high fuel pressures required. This puts even more emphasis on finer filtration of the fuel, normally to micron contaminant levels, before it is admitted to the fuel injection pump.
One of the known difficulties associated with the filtration of diesel fuel is associated with the fuel's natural tendency to nucleate wax crystals as the ambient temperature is reduced. This may be of little concern to the engine or vehicle operator until a certain temperature is reached. At this temperature, the volume of crystallized wax in the fuel may be enough to effectively block the fuel filter as if it were solid debris, thus rendering the engine inoperative. This temperature may be established for different fuels by a standard test and is called the Cold (Fuel) Filter Plugging Point or CFPP. Modern fuels, such as those formulated for use in northern Europe and Scandinavia, contain chemical additives that help the fuel to stay fluid down to low temperatures during the winter months (e.g. −34° C. in Finland and −15° C. in the UK). Other parts of the world are often not so well served and engine operation at low temperature may become affected by pressure drops in the fuel system caused by waxing.
Hitherto, it has been proposed to heat critical parts of the system, such as the housing of the filter element and fuel lines to enable an engine to start at low temperatures. Conventional heated diesel fuel filters are found to have either nickel chromium resistance wire heaters or PTC heaters (high power ceramic heaters with a positive temperature coefficient of resistance) built into the housing of the fuel filter. Such an arrangement is disclosed in EP-A-0 400 223.